Microwave ovens are well known devices for quickly and conveniently heating foods. However, microwave cooking is known to be unsatisfactory for a variety of food items, and particularly for food items requiring browning or crisping by surface heating. Microwave cooking relies upon dielectric heating of foods responsive to microwave radiation; thus, the heating characteristics in a microwave oven for some food products are dramatically different from those experienced in a conventional oven. Additionally, the use of microwave ovens can result in undesirable temperature differentials for a variety of food products. For example, some food products, when cooked in a microwave oven, will heat to a greater extent on the interior of the product rather than on the surface as a result of the dielectric microwave heating which favors heating of the product interior. This effect can be contrasted with the results of cooking in a conventional oven in which crisping or browning is achieved by exterior heating of the food item.
Furthermore, an additional problem encountered with microwave cooking is the migration of moisture contained on the interior of the food product. Specifically, in many food products, microwave radiation causes moisture contained on the interior of the food product to migrate to the product surface during cooking. The resulting food product is often left with a soggy surface that is generally undesirable in that it imparts an unsatisfactory texture and taste to the food product.
The above problems are well known in the art of microwave cooking and numerous attempts have been made to solve them. For example, various packages for microwaveable food include susceptor which undergo an increase in temperature in response to microwave radiation. Such microwave susceptors generally comprise a thin metal electrode, usually aluminum, deposited upon the surface of a substrate. The surface resistivity of these susceptors is typically in the range of about 10-500 ohms per square.
The film susceptors described above often suffer performance and physical deterioration when exposed to microwave radiation. This deterioration is a result of very rapid heating which occurs during the early stage of the heating cycle. This heating causes the substrate to undergo dimensional changes which damages the metal electrode on the substrate surface. When this electrode damage and deterioration occurs, these susceptors become less reflective, more transmissive and less absorptive to microwave radiation during heating in the microwave oven and, in so doing, experience dramatic, uncontrolled changes in heating performance. Additionally, such uncontrolled susceptors have raised concerns about chemicals leaching from the substrate or adhesives thereon into the food product, as a result of the high temperatures which occur in the susceptor during the early stages of the heating cycle. Thus, it would be very desirable to provide susceptors in which microwave power is reflected, transmitted and absorbed in a predetermined combination and which would have better temperature stability, lower temperature performances, and improved consistency in transmission, reflectivity and absorption throughout its exposure to microwave radiation during cooking.
Among the susceptors that are currently used in microwave cooking, a number of other significant problems exist. For example, current susceptors often result in a nonuniform heating of the food product. Such susceptors often produce a food product that is overheated and overcooked in some regions and underheated and uncooked in other regions. Such problems are particularly evident in large food items such as pizzas, pies, turnovers and the like.
Furthermore, prior art susceptors generally suffer from a lack of control of temperature of the susceptor itself. In prior art susceptors, the only controllable variables to effect the temperature rise have been the initial resistivity of the susceptor electrode and the substrate material. However, when subjected to microwave radiation, such susceptors are known to undergo physical changes resulting from the heating of the susceptor which causes the substrate to shrink or expand with the result that the metal electrode on the substrate surface is damaged. The damage of the metal electrode layer on the susceptor surface results in a significant decrease in microwave energy absorption and a corresponding increase in susceptor transmission of microwave radiation, thereby resulting in a significant decrease in the heating performance of the susceptor upon the outside of the food product combined with increased induction heating performance resulting from the increase in microwave transmission into the food product.
At least one successful attempt has been made in the prior art to control the damage to the susceptor and to enhance susceptor performance by using the susceptor in combination with a separate, reflecting grid to control the amount of microwave radiation that reaches the susceptor surface. This attempt is described in detail in U.S. Pat. No. 4,927,991 to Wendt, et al. the disclosure of which is incorporated herein by reference. Although addressing a number of significant issues relating to the deficiencies of prior art microwave susceptors, the product described in the Wendt et al. patent suffers by requiring a very costly and complex addition to the microwaveable food package and by creating an inherent arcing possibility at the site of any nicks or sharp edges in such reflecting foil grids.
Thus, a significant need exists for a simple, inexpensive microwave susceptor for use in food packaging that will heat evenly in a predictable manner and will experience no more than minimal deterioration when subjected to microwave radiation during the cooking cycle.